Auto transormer, 220v/110v, 110v/220v
An autotransformer (sometimes called autostep down transformer) is an electrical transformer with only one winding. The "auto" prefix refers to the single coil acting on itself and not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as both the primary and secondary sides of the transformer. In contrast, an ordinary transformer has separate primary and secondary windings which are not connected.
The winding has at least three taps where electrical connections are made. Since part of the winding does"double duty", autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but the disadvantage of not providing electrical isolation. Other advantages ofautotransformers include lower leakage reactance, lower losses, lower excitation current, and increase KVA rating for a given size and mass.
Autotransformers are often used to step up or step down voltages in the 110-115-120V range and voltages in the 220-230-240V range-for example, providing 110V or 120V (with taps) from 230V input, allowing equipment designed for 100 or 120V to be used with a 230V supply. This allows us electrical equipment to be fed from the higher European voltage. Autotransformers can also be used to supply European 230V appliances from a 100 or 120V supply in countries outside Europe. In all cases the supply and the autotransformer must be correctly rated to supply the required power.